1. Field of the Invention
The present invention relates to a microwave oven food temperature sensing probe assembly which prevents overcooking of the food in a region immediately adjacent the probe. The present invention is an improvement of the probe assembly disclosed and claimed in copending application Ser. No. 643,092, filed Dec. 22, 1975, concurrently herewith, by Flora L. Meek and Richard E. Hornung, entitled "Microwave Oven Food Temperature Sensing Probe Assembly Including a Reflector," and assigned to the same assignee as the present invention.
2. Description of the Prior Art
Electric thermometer probes which monitor the internal temperature of the food while it is being cooked have been developed for use in microwave ovens. Examples of such probes, adapted for use in microwave ovens, are disclosed and claimed in copending application Ser. No. 616,049, filed Sept. 23, 1975, by David Y. Chen and Louis H. Fitzmayer, and in copending application Ser. No. 616,050, filed Sept. 23, 1975, by Louis H. Fitzmayer, each entitled "Food Thermometer for Microwave Oven," and assigned to the same assignee as the present invention. Such probes comprising a needle-like probe adapted to be driven into the food and a temperature-sensing element, such as a thermistor, positioned internally of the probe housing near the distal end thereof. A flexible shielded cable connects the thermistor to circuitry which is responsive to thermally-induced changes in its resistance. The cable shield is electrically connected at one end of the probe housing and at the other end to a wall of the oven cooking cavity. In the embodiments disclosed in the above-mentioned Chen and Fitzmayer application Ser. No. 616,049, the total effective electrical length of the probe and cable, measured along the cable and probe from the cooking cavity wall to the distal end of the probe, is selected to be approximately equal to n.lambda./2, where n is any integer and .lambda. is the wavelength of the microwaves being used to cook the food in the oven. Such a probe and cable assembly may successfully be operated in a microwave oven with minimal direct heating of the probe and cable by the microwave energy.
In general, probes of the above-described type operate satisfactorily and do minimize problems caused by direct heating of the probe and cable by the microwave energy. However, when they are used to monitor the internal temperature of certain foods, including meats and baked potatoes, overcooking of the food in a cone-shaped region immediately adjacent the probe (generally along the length of the probe) sometimes occurs. This is considered undesirable from the standpoint of appearance of the food after it has been cooked. Further, the temperature-sensing probe necessarily responds to the temperature of the food in the region immediately adjacent the probe. When the food in this immediately adjacent region cooks faster than the other regions of the food, there is an indication of doneness before the bulk of the food (all the other regions within the food) is fully cooked to the desired degree of doneness. The condition which results in overcooking of the food in the region immediately adjacent the probe has been termed "feed in" because the cable and probe seemingly pick up microwave energy present within the cooking cavity and feed or direct the energy into the food along the length of the probe.
The above-described "feed in" effect is to be distinguished from a premature indication of doneness due to direct heating of the probe by the microwave energy, one of the problems to which the inventions comprising the subject matter of the above-mentioned copending Chen and Fitzmayer application Ser. No. 616,049 and the abovementioned Fitzmayer application Ser. No. 616,050 are directed. When a "feed in" problem occurs, the probe may be accurately sensing temperature; the temperature of the food which the probe is sensing is simply not the temperature of the food as a whole.
The above-mentioned Meek and Hornung application Ser. No. 643,092 discloses and claims a microwave oven food temperature-sensing probe assembly which, in order to eliminate the "feed in" problem, includes a disc-shaped reflector positioned along the probe housing. The reflector-to-food spacing can be adjusted to achieve a condition in which an optimum amount of microwave energy enters the food in the region immediately adjacent the probe housing. If the reflector is not sufficiently close to the surface of the food, insufficient shielding takes place and the "feed in" problem returns. By the present invention, there is provided an improved form of the Meek and Hornung probe and the reflector assembly which is convenient to use and not prone to being improperly adjusted.